1. Field of the Invention
The present invention relates to an image forming apparatus to develop an image using a two-component developer, and particularly, to an image forming apparatus capable of controlling a linear velocity ratio of a photoconductive drum and a developing roller.
2. Description of the Related Art
A general image forming apparatus, such as electrophotographic laser printer, forms an electrostatic latent image on a photoconductive medium (drum) using an exposure optical system, forms a toner image by developing the electrostatic latent image with a developing apparatus, and fuses the toner image transferred onto a recording paper.
The developer used for the developing apparatus is divided into a one-component developer and a two-component developer. In a case of the one-component developer, the toner particles are electrified by friction among themselves or by friction with a proper electrification member.
The two-component developer is a mixture of magnetic carrier particles and synthetic resin nonmagnetic toner particles, being mixed in a proper ratio. The toner particles are electrified while being mixed with the carrier particles. Thus, the electrified toner particles are transferred to a developing roller together with the carrier, and then adhere to an electrostatic latent image area on a surface of the photoconductive medium to form the toner image.
Meanwhile, a developing method using the two-component developer has been developed. A developing apparatus using the two-component developer comprises a photoconductive drum, a developing roller (a magnet roller) which rotates while maintaining a predetermined developing gap with the photoconductive drum, a doctor blade for cutting the two-component developer attached to a surface of the developing roller in a certain thickness, and a mixer for mixing the two-component developer.
In the above structure, the mixing ratio of the toner to the carrier (T/C) in the two-component developer is less than 5%. That is, if a high-speed printing is performed in a higher T/C toner ratio than 5%, the toner particles scatters and become afloat inside of the image forming apparatus, and therefore, peripheral parts beomces contaminated.
If the T/C toner ratio is decreased, an image density is accordingly decreased. In order to compensate for this problem and to implement a desired image density, in general, the developing apparatus is designed such that a ratio of a linear velocity of the photoconductive drum to a linear velocity of the developing roller is not less than 1 to 2.5. That is, the linear velocity ratio of the developing roller to the photoconductive drum is increased so that the T/C toner ratio is controlled to remain low. Then, during the high-speed printing, the image density can be prevented from decreasing.
However, in the above developing system, if the linear velocity ratio of the developing roller to the photoconductive drum is increased to much, a back portion of the toner image can be torn off with respect to a rotating direction of the photoconductive drum at a certain-thickness font, which is so-called a ‘brush mark’.
More specifically, as shown in FIG. 1, a toner T adhering to a carrier C, which is electrified to a ‘+’ potential at the developing roller, is transferred to a toner image area 2a of the photoconductive drum 2, which is electrified to a ‘−’ potential, to form the toner image. At this time, if a linear velocity of the developing roller 1 to the photoconductive drum 2 is increased, a ‘−’ potential of a non-image area 2b of the photoconductive drum 2 is increased by a heat increase of the photoconductive drum 2. Furthermore, the ‘+’ potential of the carrier C on the developing roller 1 increases. Accordingly, the carrier C on the developing roller 1 is pulled to the non-image area 2b of the photoconductive drum 2, which has an increased ‘−’ potential. In this process, the carrier C is mashed and scattered about a back portion of the toner T attached to the image area 2a (‘A’ area in FIG. 1).